Overhaul process for aluminide coated gas turbine engine components



United States Patent Office 3,544,348 Patented Dec. 1, 1970 3,544 348OVERHAUL PROCESS FOR ALUMINIDE COATED GAS TURBINE ENGINE COMPONENTSDonald H. Boone and George W. Goward, North Haven, Conn., assignors toUnited Aircraft Corporation, East Hartford, Conn., a corporation ofDelaware No Drawing. Filed Oct. 25, 1968, Ser. No. 770,853 Int. Cl. B23p7/00; C23c 9/02 US. Cl. 117-2 4 Claims ABSTRACT OF THE DISCLOSURE Anoverhaul process for coated gas turbine engine components is describedwherein the components are: removed from the engine at or prior to theonset of significant coating penetration as evidenced by the loss of oneor more substrate components; recoated in a pack cementation process ofhigh aluminum activity without stripping of the old coating; and aresubsequently heat treated to promote the formation of aluminides havingan aluminum content less than that corresponding to Ni Al BACKGROUND OFTHE INVENTION The present invention relates in general to the generationof oxidation-resistant aluminide coatings on the nickel-basesuperalloys, particularly in the overhaul and repair of coated gasturbine engine components.

It is the standard practice in the gas turbine engine industry toestablish inspection, repair and overhaul cycles for the various enginemodels on a statistical basis which is related to the projectedlifetimes of selected engine components. While a number of factors areinvolved in determining the length of such cycles and in the past thefactors have involved various mechanical properties such as fatigue orcreep of the substrate alloy, at the higher operating temperatures of anumber of the advanced engines the components are very oftencoatinglimited which means that in many cases it is deterioration of thecoating rather than accumulated mechanical stress on the part whichdetermines the duration of the overhaul cycle. It also means that, ifthe coating can be restored, the part can be salvaged and reused,possibly several times. This is of particular advantage in the case ofcertain advanced thin-walled hardware incorporating air cooling wherethe expense involved in the initial production of the component makessalvage almost mandatory, if feasible.

The present practice with salvageable parts following a coatingdeterioration or breakdown is to: remove all of the original coating;rework the part as necessary; and then recoat utilizing one of thestandard techniques such as slurry-diffusion. These current salvageoperations are not without drawback however. Most aluminide coatings areproduced by or involve a diffusion mechanism whereby the coating isformed by reaction of the coating material with the substrate elements.Therefore, the formation of the usual 3 mil coating involves consumptionof at least 2 mils of the substrate alloy. Thus, the removal of the oldcoating, even from areas where a coating failure has not occurred,necessarily results in the loss of some substrate material and areduction in the thickness of the part. While the loss of substratematerial is of some, although often lesser, concern in the case of thickwalled components, the loss of 2 mils or more of thickness on the moreadvanced hardware, some sections being but 15-20 mils thick, isintolerable. Furthermore, there are the additional problems involvingthe time and expense related to the stripping operation, not to mentionthe difiiculty of a uniform or controlled material removal. Because ofthese factors, there is an unacceptably high scrap rate associated withthe utilization of the current overhaul and repair techniques.

SUMMARY OF THE INVENTION It is a primary object of the present inventionto provide an improved overhaul and repair procedure foraluminide-coated superalloy components which have been exposed to a hightemperature oxidizing environment.

The above and other objects and advantages of the invention are providedby processing the part involved prior to excessive coating degradationor failure, in a pack cementation aluminizing process utilizing a packmix of high aluminum activity at the coating temperature, withoutstripping the prior coating therefrom.

A particularly preferred process comprises the steps of: removing thepart to be processed from its oxidizing environment prior to the onsetof any substantial oxidative attack on the superalloy substrateelements; superficially cleaning the oxide layer therefrom; recoatingthe part in a pack aluminizing process utliizing a pack mix ofsulficiently high aluminum activity at the coating temperature topreferentially form the nickel aluminides having an aluminum contentequal to or greater than that corresponding to Ni Al and subsequentlyheat treating the coated components to cause further diffusion therebyconverting at least a portion of the aluminides to those having analuminum content less than that of the Ni Al phase.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Diffused aluminum coatings forthe protection of various metals from high temperature oxidation havebeen in use for over fifty years. Two major processes are generally usedfor the application of such coatings to nickelbase gas turbine hardware.The first involves covering the surface of the metal to be protectedwith a slurry of aluminum in a liquid vehicle, followed by drying andfiring at an elevated temperature. The second process comprises thesteps of embedding the article in a dry powder mix of aluminum, an inertfiller such as powdered alumina, and an activator such as ammoniumchloride, and heating the pack to some elevated temperature for a periodof time suflicient to form a coating of the desired thickness. Thislatter process is typically referred to as pack aluminizing or packcementation aluminizing. While various other elements may be added tothe pack mix either as rate-controllers or to impart some additionalspecific property to the coating, nevertheless all such coatings consistprimarily of intermetallic compounds, such as nickel aluminides derivedfrom the aluminum in the pack and elements from the substrate and fromwhich the basic oxidation resistance is derived.

In general, the thickness, composition and structure of a packcementation coating are determined by the follow; ing controllablevariables: (a) pack mix composition; (b) processing temperatures; (c)time at temperature; and (d), any subsequent heat treatment of thecoated component. Historically, the pack cementation processes have beenperformed in large retorts necessitating the use of long times attemperature to obtain thermal equilibrium To prevent large differencesin coating thicknesses as a result of different thermal histories insuch large containers, a pack mix of low time-sensitivity is generallyemployed, such a mix being characterized by a low aluminum activity.

It has now been discovered that, if the aluminum activity of the packmix is such that the equilibrium coating phase is NiAl or other phaseslower in aluminum content, then the coating will form only by outwarddiffusion of nickel from the substrate and the phases will form on topof the substrate alloy. Virtually no aluminum motion is 3 involved inthe formation of sucha coating. While this is an unusual diffusionphenomenon, it has nevertheless been experimentally established. On theother hand, if the aluminum activity of the pack mix is such that theequilibrium coating phase is Ni Al or phases higher in aluminum content,then the coating will form only byan inward diffusion of aluminum andvirtually no nickel motion is involved in the formation of such acoating. Again, this is an unusual and unexpected diffusion phenomenon,but nevertheless experimentally established.

For mechanical property reasons, the coating comprising the Ni Al phasegenerated-in a pack of high aluminum activity cannot be practicallyemployed in many cases because of its brittleness. Therefore, the coatedalloy is normally subsequently heat treated to cause further diffusionto occur which promotes the formation of the more ductile NiAl phase.Because the driving force of high aluminum activity is no longerpresent, nickel diffusion from the substrate now occurs in combinationwith diffusion of the aluminum from the Ni Al phase to form a layer ofNiAl beneath the original Ni Al coating.

The aluminide coatings derive their protectivity from the intermetalliccompounds of aluminum which in turn are protected by a thin layer ofaluminum oxide formed by high temperature oxidation of the coating.Gradually, however, the oxide is lost by a process of erosive spalling,but a reoxidation occurs and the protective function is reestablished.Accordingly, the substrate remains protected as long as sufiicientaluminum is retained in the coating to provide for the preferentialoxidation to aluminum oxide, and a coating failure or rapid degradationoccurs when one or more of the substrate metals commences to oxidize.The protective function of the coating is, hence, a direct function ofits aluminum content.

A major factor contributing to the success of the present inventioninvolves recognition of the fact that aluminide coatings may be formedon the superalloys by an inward diffusion of aluminum provided thealuminum activity of the pack mix is sufiiciently high to formequilibrium aluminide phases of high aluminum content as previouslydescribed. For such a pack aluminizing process there is a parabolicrelationship between coating thickness and time. Thus, a part with nocoating in an eroded zone and a retained coating of 3 mils in a coolerzone will, during such a recoating process, form a new coating of Ni Alor phases of higher aluminum content, of the required thickness in allzones but in so doing will absorb considerably less aluminum in theundegraded zones than in the degraded or eroded zone. This occursbecause in the undegraded zones the aluminum content, before recoatingamounts to about 30%, corresponding to the ,3 (NiAl) phase, whereas inthe degraded zone the aluminum content may be as low as 5%. That is tosay, that in being transformed to the Ni A1 phase, which contains 40%aluminum, the undegraded zones absorb the equivalent of about aluminumwhile the degraded zone absorbs as much as the equivalent of 35%aluminum. Upon subsequent heat treatment to form the desired ductile fl(NiAl) phase, as previously described, the coating on the previouslyundegraded zones exhibits minimal new growth because these zones haveabsorbed relatively small amounts of aluminum during the recoatingprocess, whereas a completely new coating has formed on the previouslydegraded zone because this zone has absorbed a relatively large amountof aluminum. The net result is that new coating of the requiredthickness has been formed on the previously eroded area of the part withinsignificant increase in coating thickness in the cooler, undegradedzones thereby eliminating the necessity of performing the expensive andotherwise undesirable stripping operation of the current processeswherein the old coating is totally removed.

While a number of gas turbine engine components can be satisfactorilyrecoated as herein described even in those instances where there hasbeen substantial corrosive attack on the substrate metal, maximumusefulness of the present process occurs if the overhaul period selectedallows recoating of the components prior to a substantial loss ofsubstrate components by oxidation-erosion. This substrate loss occurswhen the aluminum con tent of the coating fall below that level at whichthe oxidation mechanism is preferentially toward the formation ofaluminum oxide as evidenced, in the nickel-base systems, by theoccurrence of macroscopic amounts of the blue-green, nickel-rich oxide.

The particular pack aluminizing process preferred in the presentoverhaul and repair process is one selected to provide for theincorporation of the desirable substrate alloying elements in thecoating while minimizing the formation of deleterious phases. In theprocess, the article to be coated is embedded in a pack mix containing5-20 Weight percent aluminum, 0.5-3 percent ammonium chloride, balancealumina. The pack is then heated to a relatively low temperature1200-1600 F. in an inert atmosphere and coating growth is allowed toproceed for 1-4 hours. In the recoating process a minimum coatingthickness of 0.003 inch is effected. Subsequenfly, the recoated articleis subjected to a ductilizing heat treatment in the temperature range ofl900-2200 F. usually matched to the strengthening heattreatmentsspecified for the superalloy substrate.

The exact pack composition and coating parameters employed in a givencase are, of course, dependent upon the particular component beingprocessed and the particular end result desired. In each case, however,the pack of the present invention will be characterized by a highaluminum activity, and will tend to yield coatings high in aluminumcontent for a given time at temperature. Those skilled in the art willreadily recognize the variety of alternative techniques and compositionsadapted to provide the requisite high aluminum activity.

As used herein, the term nickel-base superalloys will be understood tohave reference to those multiphase alloys of the 'y'y' type which arecharacterized by high strengths at temperatures of 1500" F. and higher.Several represeriative alloys of this type are listed in the followingta e.

In one test an erosion bar formed of the B-1900 alloy was coated byslurry techniques to a thickness of about 3 mils and run in anoxidation-erosion environment at 2100 F. for hours. The bar was removedfrom test at the first sign of coating penetration and substrateoxidation as evidenced by the appearance of the blue-green, nickel-richoxide. The specimen was sectioned through the eroded zone and one halfwas examined metallographically as a control specimen while the otherhalf was recoated in a pack of high aluminum activity for 1 /2 hours at1400 F., followed by a heat treatment of 4 hours at 2000 F. Thisspecimen was then examined metallographically to determine coatinguniformity, thickness and structure.

Examination of the control specimen revealed pitting through the coatingat the eroded zone which resulted in the formation of the blue-greenoxide. The uneroded portion of the coating in the hot zone of barcontained no ,0 (NiAl) aluminide and a thin layer of carbides wasobserved to be in the process of resolutioning in the 'y (nickel solidsolution)-'y (Ni Al) phases of the coating layer. The recoating processuniformly covered the defect area and the remaining 'y'y' phase layerwith but a slight depression in the coating surface at the eroded zone,but otherwise little difference was noted in the coating structure orthickness after recoating. The cooler zones of specimen before recoatingshowed some effects of aluminum depletion with the presence of the 7'phase of the aluminide at the grain boundaries of the ,9 (NiAl)aluminide. The recoated structure after diffusion heat treatment was allaluminide with a slightly different carbide morphology than in theoriginal specimen, but no unusual or detrimental phases were present. Asindicated by the absence of a hyper-stoichiometric aluminum rich ,6phase layer, the recoated structure was not excessive high in aluminumcontent.

A second B-1900 alloy erosion bar was coated in a process similar tothat used with the first specimen and was run in an oxidation-erosiontest at 2100 F. Although the specimen failed at about 50 hours bypitting through the coating to the substrate, the test was run to 100hours. As a result, some excessive substrate damage in hot zone on thetrailing surface was noted because of the excessive time in test.However, the specimen was lightly cleaned with a vapor blast andrecoated by a pack aluminizing process of high aluminum activity. Thisrecoated specimen was then retested for an additional 60 hours at 2100F. until coating failure again occurred.

The specimen was again cleaned and recoated, this time to produce acoating of 3 /2 mils thickness. A third oxidation erosion cycle at 2100F. was run and coating failure did not occur for an additional 80 hours.

After these tests the following observations were made with respect tothe specimen: (1) the specimen surface was still smooth and free fromany indication of defects; (2) a slight depression of 3-5 mils on thesurface of the hot zone was present, the result, however, of testing ofthe specimen beyond the limit of the coating protection in the initialoxidation-erosion test which resulted in some substrate attack; (3) thelfetime of the nickelbase substrate was successfully extended to 3 ormore times its normal useful level; (4) no excessive buildup of thecoating occurred in the previously undegraded areas and no masking wasrequired; (5) the problems associated with the stripping of the coatingwere eliminated.

It is evident that aluminide-coated alloy systems can be recoatedseveral times by the disclosed pack cementation process without thenecessity of prior coating stripping. The principal considerations arethe removal of the specimen from its oxidizing environment prior tocoating breakthrough and significant substrate attack, and the use of apack aluminizing process utilizing a pack mix of high aluminum activity.

'In view of the continuing upward trend in engine operating conditions,the increased use of thin-walled components is anticipated with theconsequent increased component cost resultant from the added complexity.Furthermore, the trend toward higher temperatures clearly forecasts thata number of engine components will continue to be coating-limited. Thus,the salvage of these expensive components will continue to be mandatoryand the overhaul and repair procedure described herein should find wideutility in such salvage operations.

While the present invention has been described in connection withcertain examples and preferred process compositions and parameters,these will be understood to be illustrative only and numerousmodifications will be 6 evident to those skilled in the art from thedetailed description. The invention in its broader aspects is notlimited to the specific details shown and described but departures maybe made from such details without departing from the principles of theinvention and Without sacrificing its chief advantages.

What is claimed is:

1. An overhaul and repair procedure for aluminidecoated gas turbineengine components formed from the nickel-base superalloys whichcomprises:

removing the oxidized components from the engine prior to the onset ofsubstantial oxidative attack on the superalloy substrate;

superficially cleaning the surface of the coated components to removethe accumulated oxide layer therefrom; recoating the components in apack aluminizing process utilizing a pack mix having an aluminumactivity at the coating temperature sufiicient to preferentially formequilibrium aluminides having an aluminum content not less than thatcorresponding to Ni A1 and diffusion heat treating the recoatedcomponents to convert the aluminides formed in the aluminizing processto aluminides having an aluminum content substantially corresponding toNiAl.

2. An overhaul and repair procedure for aluminide coated gas turbineengine components formed from the nickel-base superalloys whichcomprises:

removing the oxidized components from the engine prior to the firstappearance of macroscopic amounts of blue-green nickel-rich oxide;

superficially cleaning the surface of the coated components to removethe accumulated oxide layer therefrom;

recoating the components in a pack aluminizing process utilizing a packmix having an aluminum activity at the coating temperature sufficientlyhigh to promote formation of the coating by an inward diffusion ofaluminum toward the superalloy substrate;

and, after removal from the pack, diffusion heat treating the recoatedcomponents to preferentially form aluminides having an aluminum contentsubstantially corresponding to NiAl.

3. The procedure according to claim 2 wherein the final diffusion heattreatment is conducted in the temperature range of l900-2200 F.

4. The procedure according to claim 3 wherein in the recoating process aminimum coating thickness of 0.003 inch is effected.

References Cited UNITED STATES PATENTS 3,079,276 2/1963 Puyear et al.117 107.2 (P) 3,129,069 4/1964 Hanink et al. 29-197 X 3,141,744 7/1964Couch et a1 29 197 X 3,257,230 6/1966 Wachtelletal. 117 107.2 (P)3,345,197 10/1967 Martini et al. 117-1072 (P)X 3,436,249 4/1969 Lambertet al. 117-1072 (P) 3,450,512 6/1969 Maxwell 29-197 X ALFRED L. LEAVITT,Primary Examiner I. R. BATTEN, 111., Assistant Examiner U.S. Cl. X.R.

29197; ll77l, 107.2

